What Is a Fleece Jacket? | Warmth Without the Weight

A fleece jacket is a synthetic mid-layer made from polyester, engineered to trap body heat in brushed fibers while remaining breathable and quick-drying.

When the temperature drops but you still need to move, a fleece jacket hits the sweet spot between a base layer and a heavy coat. Invented in 1979 by Malden Mills (now Polartec), fleece was designed to imitate wool without the weight or slow drying time. The fabric works by trapping body heat in tiny air pockets within its brushed polyester fibers, and because polyester holds less than 1% of its weight in moisture, fleece dries far faster than cotton or wool. The catch is that fleece alone offers almost no wind or water resistance — which is why it earns its keep as a mid-layer under a shell, not as a standalone outer jacket.

How Fleece Weights Change What You Get

Fleece is measured in grams per square meter (GSM), and GSM is the single most useful number when picking one. Higher GSM means denser, warmer fabric; lower GSM means more breathability and compressibility. Here is the breakdown:

If you pick a heavyweight fleece for a trail run, you will overheat within minutes. If you pick a microfleece for a wind-whipped ridge in January, you will freeze. The GSM chart is the cheat sheet for getting it right.

What Modern Fleeces Are Made From

The standard fleece is 100% polyester, but brands now blend in other fibers for specific performance. Mixing polyester with 15–40% Merino wool boosts warmth and fights odor buildup — synthetic fibers tend to smell faster than natural wool during heavy activity. Adding 6–10% spandex or lycra gives the fabric four-way stretch, which is why Polartec Power Stretch can work as a close first layer in extreme cold. Most fleeces sold today use recycled polyester, a shift driven by sustainability goals that also reduces pilling. Polartec, the original inventor, still sets the benchmark with its numbered system: Micro, 100, 200, and 300 fleeces correspond directly to GSM ranges and make picking by weight simple.

How To Care For A Fleece Jacket (So It Lasts)

Fleece is low-maintenance, but one mistake ruins it fast. Wash on a gentle cycle with cold water and skip fabric softener entirely — softeners coat the fibers, destroy the hydrophobic properties that make fleece quick-drying, and accelerate pilling. Air drying is best; high heat in a dryer creates permanent fuzz. Wash after heavy activity to remove body oils that cling to synthetic fibers. For the best-buy options for smaller family members, our baby fleece jacket roundup covers the top-rated picks for warmth and durability. The North Face care guide on fleece confirms all of this and adds that dry cleaning is unnecessary — machine washing is the standard.

FAQs

Can a fleece jacket replace a winter coat?

No — fleece is a mid-layer, not an outer shell. Standard fleece has almost no wind or water resistance, so wind cuts right through it and rain soaks it. Wear it under a waterproof shell in wet or windy conditions.

Why does my fleece smell after exercise?

Synthetic polyester fibers trap bacteria that produce odor faster than natural wool does. Wash with a sport-specific detergent after hard activity, or choose a fleece blended with Merino wool for better odor control.

What is the difference between fleece and polar fleece?

“Polar fleece” generally refers to heavyweight, high-pile fleece (300+ GSM) with a deep plush texture — the warmest and least breathable type. Standard fleece covers the full GSM range from ultralight to heavyweight.